Using the Epic Model to Assess Cover Crop Effects Across a Range of Climate, Soil, and Management Conditions.

Contamination of surface and groundwater with nitrate-nitrogen (N) has been linked to agricultural production in several regions of U.S.  In the early 1980’s degraded water quality in Chesapeake Bay was linked to excessive N inputs, with agriculture being a major contributor. Nitrate leaching into shallow groundwater and subsequent discharge into surface waters has been identified as the dominant route of N delivery from cropland in many Chesapeake Bay sub-watersheds.  In watersheds where agriculture is the dominant land use, meeting total N reduction goals for Bay restoration will require major reductions in rates of nitrate leaching.  One practice shown to be highly effective at the field scale for reducing nitrate leaching following corn and soybean production is the use of winter cereal grain cover crops.  Nitrate leaching tends to be greatest during winter when evapotranspiration rates are low, allowing a fall window for cover crops to reduce soil nitrate pools before leaching occurs. However, with varying cover crop planting times, soil water holding characteristics and autumn temperature and precipitation, the effectiveness of cover crops for reducing nitrate leaching can vary from year to year in the same location, and across small spatial scales depending on soil characteristics.  Spring cover crop management is an additional variable, since growing cover crops take up nitrate released as soils warm, but can increase spring leaching volume after herbicide application by reducing soil evaporation rates.  While field research has addressed many of these issues, it is insufficient to cover the full range of variability in effectiveness needed to estimate cover crop effects on N loads at large watershed and regional scales. Here we use the latest version of the EPIC model (EPICv1102) to test the effect of several combinations winter cover crop planting and termination dates across a range of observed weather conditions and soil characteristics. As would be expected results showed that the cover crop planting and termination dates affect N losses, but that the extent of the effect varies widely with weather and soil characteristics that alter the relative rates of leaching and cover crop N uptake.  This study lays the groundwork for assessing annual weather driven variability in cover crop effectiveness for reducing cropland N losses across watersheds with varying soil characteristics, and for developing management approaches needed to meet long-term N reduction goals.